European and Asian viruses within the tick-borne encephalitis flavivirus complex are known to show temporal, spatial and phylogenetic relationships that imply a clinal pattern of evolution. However, the isolation of recognized Far-Eastern tick-borne encephalitis virus (TBEV) strains in the European region of the former Soviet Union (SU), i.e. thousands of kilometres west of the region in which they are considered endemic, appears to contradict this concept. Here, we present a parsimonious explanation for this apparent anomaly based on analysis of the dates and regions in which these non-endemic strains were isolated, together with their phylogenetic relationships and the records of redistribution of animals under the All-Union programme for acclimatization of game animals within the former SU. Our evidence supports the concept that the anomalous distribution of Far-Eastern TBEV strains in Europe and Siberia arose primarily as the result of the large-scale westward redistribution of game animals for economic purposes.Tick-borne encephalitis virus (TBEV), belonging to the tickborne flavivirus group, genus Flavivirus, family Flaviviridae, is a prototype representative of the seroviruses group of the same name, which were discovered in 1937 in the Russian Far East. TBEV is the causative agent of tick-borne encephalitis in humans, usually after the bite of an infected tick. Two types of host are required for TBEV circulation in nature. The first is the tick as the reservoir and carrier of TBEV and the second the vertebrate animal whose blood is the nutrient source for ticks and also the way in which the virus is transmitted from infected to non-infected ticks by their feeding on the same animal (Labuda et al., 1993). Ixodes ricinus L. and Ixodes persulcatus Schulze are the two main TBEV vectors. Small mammals are the principal hosts for pre-imaginal ticks, whereas mature ticks feed on large mammals such as lagomorphs, predators, hoofed animals and birds (Pavlovsky, 1947;Filippova, 1985). All these animals are natural reservoirs of TBEV in the infection hot spots.According to phylogenetic analysis, there are three TBEV subtypes: Far-Eastern (FE-TBEV) and Siberian (S-TBEV), which are both transmitted by I. persulcatus, and European (Eu-TBEV), which is transmitted by I. ricinus. Each TBEV subtype has specific nucleotide substitutions in the protein E gene and their classification is based on this characteristic (Ecker et al., 1999). Strains belonging to the different TBEV subtypes have their own geographical distributions. Specifically, Eu-TBEV is endemic in Europe and includes strains collected in Austria, Switzerland, Germany, Sweden, Hungary, the Czech Republic, Slovenia, Finland, Latvia, Lithuania, Estonia, Byelorussia (Belarus) and the European part of Russia (Ecker et al., 1999;Lundkvist et al., 2001;Haglund et al., 2003), FE-TBEV is distributed mainly in the Russian Far East, the eastern part of North China and northern Japan. S-TBEV is commonest in eastern and western Siberia, in the Ural region, the European pa...